1. Field of the Invention
The present invention relates generally to imaging, and in particular, to a method apparatus, system, and article of manufacture for imaging astronomical spectral regions.
2. Description of the Related Art
The sounding rocket experiment FIRE (Far-ultraviolet Imaging Rocket Experiment) illustrates the ability to image a spectral region that hitherto has been unexplored astronomically. The imaging band of FIRE (˜900-1100 Angstroms (Å) electromagnetic radiation wavelength) helps fill a wavelength imaging observation hole existing from ˜620 Å to the GALEX (Galaxy Evolution Explorer™) observed band near 1350 Å wavelength. Demonstration of the usefulness of the FIRE wavelength band provides the ability to develop/conduct space-based missions that can find and study quasars, star forming regions and galaxies and other UV (ultraviolet) bright objects.
Star formation typically produces the hottest, brightest types of stars, O stars, 30,000-50,000° C. The emission peaks of O stars are within the FIRE waveband and constitute a majority of all the observed light emitted at these wavelengths. Therefore, the 900-1100 Å band provides the most sensitive indicator of young, massive stars. Combining this information with UV and visible wavelength data from other missions and comparing the spectral colors to stellar evolution models can help with the determination of the star formation history.
Complicating an accurate determination of the stellar population of young, hot stars is the intervening dust. To determine the quantity of O stars, one must first determine the effect this dust has on the recorded spectrum. The extinction due to dust, however, varies strongly with wavelength in the UV and over differing lines-of-sight. GALEX has previously attempted this measurement and effectively gave two data points corresponding to the two wavelength bands it measured, 1350-1780 Å and 1770-2800 Å. FIRE has similar angular resolution and field-of-view as GALEX and is intended to compliment that mission with the 900-1100 Å measurement. Combined with visible light measurements, this produces four points upon which a dust extinction model can be determined. While this does not uniquely constrain extinction models, it limits the possibilities, allowing more accurate removal of the effects of the intervening dust.
FIRE is a single optic prime focus telescope with a 1.75 m focal length. The bandpass of 900-1100 Å wavelength is set by a combination of the mirror coating, the indium filter in front of the detector and the salt coating on the front of the detector's micro-channel plates. Critical to this is the indium filter that must reduce the flux from Lyman-alpha at 1216 Å by a minimum factor of 10−4. The cost of this Lyman-alpha removal is that the filter is not fully transparent at the desired wavelengths of 900-1100 Å.
Accordingly, the prior art fails to provide filters that sufficiently remove Lyman-alpha while maintaining sufficient transparency. The above example illustrates the utility of an indium filter for ultraviolet astronomy applications, but it is clear that high quality filters are useful for a variety of imaging applications for a wide variety of regions of the electromagnetic spectrum.